1. Field of the Invention
The present invention relates generally to a timer circuit used as an interval timer or the like in an electronic circuit. More particularly, the present invention relates to a timer circuit including circuitry for stretching the duration of an input pulse to a prescribed value.
2. Description of the Prior Art
In an electronic circuit, in order to produce pulses of fixed time duration, usually a one-shot timer (monostable multivibrator) is used. However, a one-shot timer only uses an input pulse as a trigger and therefore cannot be used in applications where it is required to stretch the duration of an input pulse by a prescribed time length.
For such applications, a timer circuit as shown in FIG. 5, for example, has been used heretofore which uses a pulse stretching circuit 11 to stretch the duration of an input pulse by a prescribed time length. The pulse stretching circuit 11 of this timer circuit is constructed in such a way that an input pulse and a pulse produced by delaying the input pulse by a delay time D through a delay circuit 11a are input to an OR circuit 11b. As a result, the OR circuit 11b produces an output pulse with the input pulse duration stretched by the delay time D through the delay circuit 11a, as is shown in FIG. 6.
However, when the input pulse duration is shorter than the delay time D, the output of the OR circuit 11b falls at the falling edge of the input pulse which occurs before the corresponding pulse passed through the delay circuit 11a arrives at the input of the OR circuit 11b. This results in the splitting of the output pulse provided at the output of the OR circuit 11b, as is shown in FIG. 7. Therefore, in applications where there is a possibility that the duration of an input pulse may become short or where it is required to stretch the pulse duration significantly relative to the duration of the input pulse, a timer circuit such as that which is shown in FIG. 8 has been used in the past. The timer circuit of FIG. 8 consists of a plurality of cascaded stages, each constructed with the pulse stretching circuit 11 of FIG. 5 comprising the delay circuit 11a and the OR circuit 11b. The plurality of stages are intended to produce an output pulse with its duration stretched to a desired length even when the input pulse duration is short.
However, even with the plural stage timer circuit shown in FIG. 8, when the input pulse duration is shorter than the delay time provided by the delay circuit 11a in the pulse stretching circuit 11 of each stage, it will be appreciated that the splitting of the output pulse cannot be avoided, as in the case shown in FIG. 7.
Accordingly, the prior art timer circuit has had the problem that it requires a very large number of pulse stretching circuits 11, each providing only a short delay time and increasing the size of the entire circuitry, when the input pulse duration is particularly short and when it is required to stretch the pulse duration significantly.
In view of the above situation, it is an object of the present invention to provide a timer circuit which stretches the duration of an input pulse while avoiding the splitting of the output pulse even when the input pulse duration is particularly short. It is another object of the invention to provide such a timer by including a circuit for securing a minimum value for the input pulse duration.
The present invention overcomes the aforementioned and other shortcomings of the above known and similar timer circuits which use a pulse stretching circuit to stretch the duration of an input pulse by a prescribed time length. The present invention is summarized and is described in detail below.